Serine proteases, a family of enzyme which catalyze the hydrolysis of substrate peptide bonds of neutral or slightly alkaline pH, are traditional prototypes for the study of enzyme mechanisms. We are studying structural and catalytic features of these enzymes using several tools. These features and tools include: (1) the structural and stereochemical requirements of the enzyme using structurally modified substrates and inhibitors; (2) the effect of inhibitor binding on enzyme structure by carbon-13 nuclear magnetic resonance (C-13 NMR) using C-13 enriched enzymes; (3) the role of the catalytic triad consisting of serine, histidine, and aspartate residues using C-13 NMR; and (4) changes in protonation states, protein size, and charge redistribution in the proteins in going from native protein to the transition state for the catalytic process using kinetic salt effects. Recognition of the important role which proteases play in the regulation of important physiological functions such as blood clotting, reproduction, protein turnover, and the complement system, has generated interest in their structure and mechanism.